Hitherto, a carrier which is used for imparting a proper amount of a positive or negative electrostatic charge to toner particles in a magnetic brush developing process is generally classified into a coated series carrier and an uncoated series carrier but in the case of considering the life of the developer, the former is superior and hence the former carrier has been developed and practically used. In particular, in the case of a coated series carrier applied with a resin coating, the charge-controlling property is improved and also the environmental reliance and the stability with the passage of time are improved, whereby various resin-coated carriers have been developed.
However, the most troublesome problem in the case of using a two-component developer is that the charge-controlling property is deteriorated with the passage of time due to staining of the surface of the carrier with the binder resin, a charge-controlling agent, external additives, etc.
For preventing the occurrence of the surface stain of carriers, it has been proposed to use a fluorine series polymer and a silicone series polymer each having an excellent surface staining resistance and a low surface energy.
A fluorine series polymer has a low surface energy but has faults that a large amount of the polymer cannot be used for a negative charge imparting carrier owing to the high electric negativity of a fluorine atom which is a constituting component of the fluorine series polymer, and in particular, under high temperature and high humidity conditions, the charge imparting faculty is too reduced to use for practical purpose.
On the other hand, a silicone polymer has a low surface energy and is effective for preventing the occurrence of surface staining of a carrier as the fluorine series polymer but cannot completely prevent staining. Also, a silicone polymer is also positioned at the intermediate position of the triboelectrification series and it is hard to say that the polymer is sufficient for a negative charge imparting carrier In a carrier coated with a silicone polymer, raising the electrostatic charge is delayed under a low temperature and low humidity condition and in the case of newly adding toners, low-charged toners are liable to form, which results in causing fog and scattering of the toners.
Also, in these polymers, there are problems in the points of poor adhesion with the core materials of the carriers and the strength of the coating itself
For overcoming the foregoing problems, use of an epoxy resin as an intermediate layer for improving a fluorine-containing polymer and a carrier core material is described in JP-A-49-51950 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). Further, it is proposed that the adhesion is improved by using a fluorine-series block copolymer and a graft copolymer, as described in JP-A57-99653 and JP-A-60-202451 and it is also proposed that the adhesion is improved by using a fluorine series polymer containing a hydrolyzable silyl group, as described in JP-A-2-16573 and JP-A-2-24670. In these cases, the adhesion is improved, but further improvements are yet required in the points that the carrier coated with the polymer has a low negative charge imparting faculty to toners and the environmental stability is insufficient.
Concerning the use of a silicone series polymer, it is proposed to use a coupling agent for improving the adhesion of the silicone resin coated layer, as described in JP-A-60-19156. The adhesion of the silicone resin coated layer is improved by this method. However, there is also a problem that the negative charge imparting faculty of the carrier to toners is insufficient though the fault is not so severe as the case of using the fluorine series polymer, and also the effect of preventing carrier surface staining is insufficient.
JP-A-62-121463 describes a carrier having a coated layer composed of a silicone resin formed on the surface of a carrier core treated with a silane coupling agent for improving the adhesion between the carrier core and the silicone resin. However, the coated layer composed of a silicone resin has such problems on production that nonuniform coating is formed and a high curing temperature is required.
Also, in JP-A-64-35563, JP-A-64-29857, and JP-A64-29860, there are described carriers each having a laminated layer structure but in the case of repeatedly using the carriers for a long period of time, there is a problem that each of the carriers cannot keep the electrostatically charging faculty.
Furthermore, for overcoming the foregoing problems, coated carriers each having laminated layers of a triboelectrification controlling layer and a releasable surface layer on the core material of the carrier are proposed in JP-A-61-110159 and JP-A-61-110160 and it is described therein that the carriers can stably impart negative charges to toners and can keep well the electrostatically charging faculty thereof However, in these carriers, there are such problems in practical use that the initial value electrostatic charging varies to a large extent and spreading of charge distribution occurs on toners.